1. Field of the Invention
The present invention relates to flexible polyimide materials used in thin film form that have an unusually low and tunable electrical resistivity that are useful in electrical and optical applications.
More particularly, the invention relates to flexible thin films made from polyimide materials having an adjustable number of electron-transport and/or hole-transport groups useful for tuning the electrical properties of the films, these polymers and films being useful in electrical and optical applications.
2. Description of the Related Art
Several organic compounds in thin film form having electron-transport or hole-transport properties are known. However, these organic compounds are generally limited to small molecules, for example less than 1000 Daltons in molecular weight, such as oligo phenylenes, oligo thiophenes, oligo aryl amines, and the like which form brittle films. Other common organic transport materials include small molecules that are made into films by an evaporative sublimation process, such as electron-transport materials incorporating tris(8-hydroxy)-quinoline (Alq3), or hole-transport materials incorporating triphenyldiamine derivative (TPD) (see G. E. Jabbour, et al, Elec. Lett 1997, v. 33 (24), p. 2070). Films of Alq3 and TPD must be laid down by an evaporation technique requiring expensive equipment and the film quality is difficult to reproduce (see G. E. Jabbour, et al., Appl. Phys. Lett. 1997, v. 71 (13), p. 1762; S. Tokito, et al., Appl. Phys. Lett 1997, v. 70 (15), p. 1929; R. H. Jordan, et al., Appl. Phys. Lett 1997, v. 69 (14), p. 1997; and S. A. Van Slyke, et al., Appl. Phys. Lett 1996, v. 69 (15), p. 2160). Furthermore, these small molecules tend to form crystals upon aging that change the performance characteristics of the device.
Suitable materials for flexible thin films should provide a substantially amorphous polymeric material with unusually high (and tunable) level of conductivity of electrons and/or holes, and these films additionally provide thermal stability and such characteristics as a “tunable” index of refraction, polarity, adhesion to substrates, crosslinkability for resistance to solvents, in addition to being easily processed into thin films.
Many organic materials have some, but not a sufficient number of these characteristics. One popular hole-transport material is poly(vinylcarbazole) (PVK). PVK is easily attacked by solvents used in fabricating other layers that must be placed on top of the PVK layer. Similar polymers are known to have little resistance to heat and solvent attack (e.g., E. S. Kolb, et al, Macromolecules 1996, v. 29, p. 2359). An example of a typical electron-transport polymer is poly(phenyl quinoxaline) (PPQ). PPQ suffers from solvent attack and adhesion problems. Other polymers exist which contain both hole transport and electron transport groups with high thermal stability (see J. Lu, et al., Chem. Mater. 1999, v. 11, p. 2501-2507) but their electrical resistivity properties are not known and in addition these polymers are not crosslinked, therefore they do not have the high resistance to solvents required. It is noted that tryphenylamine containing polymers are reputed to be hole transporting layer materials (see G. Liou et al., Journal of Polymer Science: Part A: Polymer Chemistry, V. 40, p. 2811) and that they may be applied in organic electroluminescent elements but no actual examples or measurements were made.
Hole transport compounds based on aromatic amine structures are described in U.S. Pat. No. 5,256,945 issued to Imai et al. on Oct. 26, 1993 but these structures are oligomeric, not polyimides and do not have the properties required for the present invention.
Structures of polymeric thin films and the process for forming the film have been disclosed in U.S. Pat. No. 5,231,329 issued to Nishikitani et al. on Jul. 27, 1993 and U.S. Pat. No. 5,540,999 issued to Yamamoto et al. on Jul. 30, 1996, incorporated herein by reference. These patents neither disclose the polyimide polymer structure of the present invention, nor do these patents disclose the advantages of the presently described structure, such as low electrical resistivity and solvent resistance.
In U.S. Pat. No. 6,232,428 issued to Deets et al. on May 15, 2001 a process is described that creates colorless and transparent polyimide coatings or films. This process does not tune polyimides for electrical conductivity or resistivity by the incorporation of specific electron transport and hole transport groups. The films created in Deets do not have the properties required of the present invention.
In U.S. Pat. No. 5,502,157 issued to Chang et al. on Mar. 26, 1996 a copolyimide is disclosed that will be an electrical insulator and contains no monomer units that would reduce electrical resistivity.
There is a need for materials that provide unusually high conductivity (and conversely, unusually low electrical resistivity) of electrons and/or holes in amorphous thin-film form that are optically transparent. The required range of electrical resistivity of the materials is from 106 to 1016 Ohm-centimeters. Additionally, it is preferable that these materials provide thermal stability with such characteristics as a “tunable” index of refraction, polarity, adhesion to substrates, crosslinkability for resistance to solvents, and that are also easily processed into thin films. The present invention addresses these needs.